A traction mechanism drive is used, for example, in a motor vehicle. Here, part of the power which is obtained in the internal combustion engine by way of the chemical conversion of the fuel is utilized to drive the auxiliary assemblies which are utilized for the operation of the internal combustion engine and the motor vehicle, in particular the injection pump (e.g., fuel injection pump), the oil pump, the coolant pump, the generator and the like or the camshafts of a valve gear which are required for the control of the valves.
Belt drives or chain drives are used for the drive, and the various drive mechanisms, such as belt and chain, being combined under the general term of traction mechanism in the context of the present invention, that is to say being subsumed under the superordinate term of traction mechanism. A traction mechanism is therefore also spoken of generally in the following text.
With as low energy losses as possible and with as little maintenance outlay as possible as a result of retensioning, the traction mechanism drive is intended to transmit a great torque from the crankshaft to the auxiliary assemblies, in particular the camshafts and the injection pump. Here, the drive of a plurality of auxiliary assemblies is frequently combined in one traction mechanism drive.
In order to keep the traction mechanism tensioned and therefore to ensure a drive which is as reliable and wear-free as possible, a tensioning device is provided at a suitable location of the traction mechanism drive, which tensioning device loads the traction mechanism with a force by way of engagement transversely with respect to the running direction, with the result that the traction mechanism is always under tension and is always kept under tension. This is indispensable for the reliable transmission of a sufficiently great torque or drive moment, in particular in order to avoid slip of the traction mechanism, that is to say, in particular, in order to ensure a slip-free drive.
Slip of the traction mechanism can also be avoided by the fact that positively locking traction mechanisms are used, that is to say chains or toothed belts; a tensioning device is also to be provided if positively locking traction mechanisms are used, in order to reliably prevent the traction mechanism from jumping over the externally toothed wheels of the traction mechanism drive.
The wear of the traction mechanism is a continuous process which is made discernible, inter alia, by the fact that the length of the traction mechanism increases continuously. A tensioning device of the described type reacts constantly during operation to said wear-induced length change of the traction mechanism and continues to keep the traction mechanism under tension despite the length change.
Tensioning devices of the described type make it possible to dispense with the retensioning in the context of maintenance measures, for example an inspection, for which reason the servicing intervals can be increased. For example, German laid-open specification DE 10 2007 025 731 A1 describes a method for detecting an elongation of an endless chain of a chain drive. German laid-open specification DE 10 2014 009 509 A1describes a method for determining a crack, that is to say a break in a belt.
Nevertheless, the inventors herein have recognized that there are further aspects and effects which have to be taken into consideration in conjunction with the use of traction mechanism drives in internal combustion engines.
As a consequence of the wear-induced length change ΔL of the traction mechanism, the position of the wheels of the traction mechanism drive changes relative to one another, in particular the position of the driven wheels relative to the driving wheel. A driven wheel rotates with respect to the driving wheel by a rotational angle σi, while the traction mechanism is experiencing the wear-induced length change.
The rotation or the positional change by the rotational angle σi has very different consequences, in a manner which is dependent on the relevant auxiliary assembly, on the shaft of which the rotated driven wheel is arranged.
If the relevant auxiliary assembly is the mechanical injection pump of the internal combustion engine, the rotation of the driven wheel which is arranged on the shaft of the injection pump and via which the injection pump is driven leads to a change in the injection parameters, in particular the injection start in ° CA. That is to say, the crankshaft and therefore the pistons which oscillate in the cylinders of the internal combustion engine no longer run synchronously with the injection pump, but rather have an undesired phase difference with respect to the injection pump and therefore with respect to the injections generated by the injection pump. This can result in disadvantages with regard to the fuel consumption and the pollutant emissions.
If the relevant auxiliary assembly is a valve gear of the internal combustion engine, the rotation of the driven wheel which is arranged on a camshaft of the valve gear and via which the camshaft is driven and is set in rotation leads to a change in the control times in ° CA. That is to say, the crankshaft and therefore the pistons which oscillate in the cylinders of the internal combustion engine no longer run synchronously with the valve gear, but rather have an undesired phase difference with respect to the valve gear and therefore with respect to the valves which are actuated by the valve gear. Disadvantages of a very different type can occur during the charge exchange, for example impaired residual gas flushing or a reduced degree of filling. In the individual case, in particular in highly compressing engines, the valves which open into the cylinder during the charge exchange can make contact with the piston, that is to say can collide with the piston which is running through the top dead center, as a result of which the functional operability of the valve gear and ultimately of the internal combustion engine can be endangered.
To this extent, it may be insufficient to react to a wear-induced length change by way of a tensioning device which merely continues to keep the traction mechanism under tension despite a length change.
Rather, the inventors herein have recognized that a method may be used by way of which the length change of the traction mechanism is monitored, in order to intervene in the case of a threshold length change which can be determined also by way of a threshold rotational angle σi of a driven wheel, optionally by way of replacement of the traction mechanism. A determined length change can also be evaluated as an indicator for an imminent crack or fracture of the traction mechanism; degradation of the traction mechanism as a result of such a crack or fracture is to be avoided under all circumstances.
Thus, the inventors herein propose a method for monitoring a traction mechanism drive of an internal combustion engine with a crankshaft, which traction mechanism drive comprises, in addition to the traction mechanism, a first driving wheel which is arranged on the crankshaft and at least one further, second driven wheel which is arranged on a shaft of an auxiliary assembly, the traction mechanism being guided around the driving first wheel and the at least one further second driven wheel, and a movable tensioning device being provided, engages in a force-loaded manner into the traction mechanism, and loads the traction mechanism with tensile forces along its longitudinal axis for the purpose of tensioning. The method includes determining the position of the movable tensioning device relative to the traction mechanism drive using measuring technology, determining an actual length LAS′ of the traction mechanism computationally using the position of the tensioning device which was determined using measuring technology, and determining a length change ΔL with respect to a predefinable setpoint length LAS computationally using the computationally determined actual length LAS′.
In this way, not only can an elongation of the traction mechanism be recognized, that is to say detected, by way of the method according to the disclosure, but also a method for monitoring a traction mechanism drive is indicated, by way of which the wear-induced length change ΔL of the traction mechanism can be computationally determined continuously.
According to the disclosure, the position of the movable tensioning device relative to the traction mechanism drive is determined using measuring technology in order to computationally determine the wear-induced length change ΔL of the traction mechanism. The awareness of the instantaneous position of the tensioning device together with the knowledge about the kinematics of the traction mechanism drive, in particular the known diameters and the known arrangement of the wheels with respect to one another, allows the actual length of the traction mechanism to be calculated. A comparison of the actual length of the traction mechanism with the setpoint length of the traction mechanism, for example the length of a new traction mechanism or of a traction mechanism which is newly mounted and tensioned in the traction mechanism drive, leads to the relevant length change which is of interest in the present case.
According to the disclosure, the computationally determined length change ΔL of the traction mechanism can be used, furthermore, to determine the rotational angle σi of a driven wheel with respect to the driving wheel. Said rotational angle σi can in turn be used according to the disclosure to influence the relevant auxiliary assemblies, for example by changing or adapting control operation, or can be used for a different type of reaction.
In another example, the instantaneous state of the traction mechanism, for example a wear-induced length change, can be monitored and/or a crack in the traction mechanism can be detected. To monitor the instantaneous state, a vibration behavior of the traction mechanism is detected metrologically, and the metrologically detected vibration behavior of the traction mechanism is evaluated using an evaluation device, in such a way that a statement is made about the state of the traction mechanism.
The method according to the disclosure for monitoring a traction mechanism drive utilizes the circumstance that both the driving crankshaft and the driven auxiliary assemblies, for example a driven valve gear, are dynamic, vibratory systems.
Together with the coupled engine parts, the crankshaft forms a vibratory system. Here, the crankshaft is excited to perform torsional vibrations by way of the torsional forces which change over time and are introduced into the crankshaft via the connecting rods which are articulated on the individual crank pins. The torsional vibrations of the crankshaft are transmitted in an undesired manner to the traction mechanism and the traction mechanism drive and therefore also to the auxiliary assemblies, for example a valve gear with associated camshaft, the camshaft itself also representing a vibratory system. The rotating camshaft is thus loaded with a torque counter to its rotational direction when the cam moves the valve in the direction of the valve open position counter to the prestressing force of a valve spring, whereas, during the closing operation when the valve is pressed in the direction of the valve closed position by the valve spring, the camshaft is loaded via the cam with an additional torque in the direction of the camshaft rotation. The torsional vibrations of the camshaft in turn influence the crankshaft or the crankshaft torsional vibration.
The method according to the disclosure for monitoring a traction mechanism drive then utilizes the circumstance that the traction mechanism per se is likewise a vibratory system or a system which vibrates during operation of the internal combustion engine and is excited to perform different vibrations.
It is utilized here according to the disclosure that the vibration behavior of the traction mechanism additionally and in a particular way depends on the state of the traction mechanism. In particular, the method according to the disclosure proceeds from the finding that a wear-induced length change in the traction mechanism or a defect of the traction mechanism has an influence on the vibration behavior and, as a consequence, leads to a change in the vibration behavior which can be detected metrologically.
The metrologically detected vibration behavior of the traction mechanism is evaluated using an evaluation device, in such a way that, proceeding from the vibration behavior, a conclusion is made about the state of the traction mechanism.
The engine controller of the internal combustion engine can serve as evaluation device. The metrologically detected vibration behavior of the traction mechanism is used as an input signal, and the state of the traction mechanism represents the output signal. Tables, characteristic maps, look-up tables and/or the like which are generated empirically on the test bench can be stored in the evaluation device, which tables, maps and the like assign a specific state of the traction mechanism, for example an instantaneous length, a length change, wear, remaining service life and/or the like to a defined vibration behavior of the traction mechanism.
A comparison of the actual length of the traction mechanism with the setpoint length of the traction mechanism, for example the length of a new traction mechanism or of a traction mechanism which is newly mounted and tensioned in the traction mechanism drive, leads to the relevant length change which is of interest in the present case. The length change ΔL of the traction mechanism can be used further.
In the individual case, a conclusion about the rotational angle σi of a driven wheel with respect to the driving wheel can be made on the basis of a wear-induced length change ΔL of the traction mechanism. Said rotational angle σi in turn can be used to influence the relevant auxiliary assemblies. The kinematics of the traction mechanism drive are known, in particular the diameters and the arrangement of the wheels with respect to one another.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.